1. Field of Invention
This invention relates generally to a drawing device having a shiftable stylus adapted to draw on the undersurface of a screen rendered opaque by powders, various line drawings or sketches whose contours depend on the operator-controlled path taken by the stylus as it scrapes powder from the screen, and more particularly to a manually-operated interrupter mechanism which when activated functions to retract the stylus from the screen undersurface without, however, inhibiting shifting of the stylus, thereby making it possible selectively to break the lines on the screen created by the stylus.
2. Status of Prior Art
The Grandjean U.S. Pat. No. 3,305,113, and the Clark U.S. Pat. No. 3,760,505, disclose a tracing device having educational as well as play value. The tracing device includes a box-like case having a transparent top plate or screen next to which are left and right control knobs. By turning these knobs, one can delineate on the screen various letters, charts, designs and other line drawings and sketches.
In a tracing device of this type, one well known version which is commercially available under the trademark Etch-A-Sketch, in order to form a horizontal line in the X-direction on the screen, the operator has only to turn the left knob, while to form a vertical line in the Y direction, he turns the right knob. And to create curves and angles on the screen, these knobs are turned at the same time, thereby causing the stylus to shift in a path which is the vector resultant of the X and Y movements. The terms "drawing" and "tracing" as used herein are interchangeable, as are the terms "shiftable" and "movable".
The case is partly filled with a slightly adhesive powder which sticks onto the undersurface of the screen to render it opaque but is easily dislodged therefrom. The undersurface of the screen is engaged by the movable stylus which under knob control scrapes the powder from the screen to define a line whose contour depends on the operator-controlled path taken by the stylus.
To erase the line drawing or sketch defined by the scraped-off powder on the screen, the case is turned upside down and is shaken to cause the powder to cascade over the undersurface of the screen, to again form an opaque coating.
The later-issued Clark patent differs from the earlier Grandjean patent mainly in the means incorporated therein to prevent leakage of powder from the tracing device. Both patents, whose entire disclosures are incorporated herein by reference, make use of transverse and longitudinal rods at whose intersection the stylus is mounted for sliding movement. The transverse rod is fastened at its extremities to parallel sections of a first string loop operatively coupled to the left control knob so that as this knob is turned, the transverse rod and the stylus borne thereby are caused to move toward either side of the box, depending on the direction of knob rotation, thereby drawing a horizontal line on the screen. The longitudinal rod is fastened at its extremities to parallel sections of a second string loop operatively coupled to the second knob so that as this knob is turned, this rod and the stylus borne thereby are caused to move toward either the top or bottom of the box, thereby drawing a vertical line on the screen.
Thus, operation of one knob produces a line in the X-direction on the screen, operation of the other knob produces a line in the Y-direction, and concurrent turning of the knobs produces a line in a Z-direction which is the vector resultant of the X and Y movements of the stylus. The drawing device is therefore capable of creating line drawings or sketches having a desired configuration.
The most serious practical limitation of the tracing device of the type disclosed in the Clark and Grandjean patents as well as in commercially-available devices based on these patents is the inability of these devices selectively to break the line drawn by the stylus. The stylus is urged against the undersurface of the screen under the tension of the string loops supporting the intersecting rods on which the stylus is mounted, and the stylus is never retracted therefrom. Because of this limitation, whatever drawing or sketch is traced on the screen by the operator must be formed by unbroken lines.
To explain why this limitation is troublesome, let us assume that a child wishes to draw with a pencil a simple picture composed of a house at ground level above which is the sun, a cloud and an airplane. These elements of the picture are all discrete and separated from each other in space. If, now, the child wishes to draw the same picture on a standard drawing device in which the stylus effectively performs the function of a pencil, he cannot on the screen of the drawing device create the elements thereof in discrete form, for the stylus never leaves the screen.
Hence, after completing, say, the house, in order to draw the sun the child must now trace a line from the house to a point thereabove before outlining the sun, and do likewise when going from the sun to a cloud, and from the cloud to an airplane. As a consequence, the picture is impaired by connecting lines which serve no artistic purpose but are dictated by the limitations of the tracing device. And when the child wishes with the tracing device to print on the screen an expression such as HAPPY BIRTHDAY, the letters cannot be separately traced but must be joined together as in cursive writing. Thus, however the knobs are manipulated, the operator can never lift the stylus from the screen as one can lift a pencil from paper.